Kentarou's former shogi teacher taught him to (metaphorically) dive into the 81 squares of the shogi board. Thus, his nickname is "81 Diver". He has failed to enter the professional league of shogi competition. However, he continues to play the game for a living by gambling at amateur shogi clubs.

Shogi's not really a game you can make a manga series around unless the main character has big breasts. Unfortunately, our protagonist is male. Boo.

Art's as atrocious as ever, but that was never the guy's selling point.

I really need to stop sounding so negative in these comments. Hachiwan Diver is a solid manga about warriors dueling it out through shogi. The complicated rules of shogi made for an easy decision on my part to never pick up the game, but my brief stint as a competitive Chess player during the 5th grade allows me partial familiarity with the concept. A majority of the manga deals with amateur "money matches" but delving further into the story reveals that these players hold very little interest in monetary gains.